In moral theology, an act is the knowing choice of a human person. Each knowing choice is an act, and each act is subject to the eternal moral law. Some acts are moral, and other acts are immoral. An immoral act is a sinful act. Sin is a knowingly chosen immoral act.
The morality of any act is based on three fonts (or sources):
(1) The intention or purpose for which the act is done,
(2) the inherent moral meaning of the act as determined by its moral object,
(3) the circumstances of the act, especially the consequences.
To be moral, each and every act must have three good fonts of morality. The intention must be good, the moral object must be good, and the good consequences must outweigh any bad consequences. If any one font is bad, the act is immoral. If an act is immoral due to a bad intention, the same type of act may be moral with a good intention. If an act is immoral due to the circumstances, the same type of act may be moral in different circumstances.

But when an act has an evil moral object, the act is inherently immoral, in other words, the act is evil, in and of itself, apart from intention and circumstances. Every intrinsically evil act has an inherent moral meaning (the moral species) which is contrary to the moral law of God. Intrinsically evil acts are never justified by intention or circumstances because the moral species (the type of act in terms of morality) is inherently unjust.

Pope John Paul II: "But the negative moral precepts, those prohibiting certain concrete actions or kinds of behavior as intrinsically evil, do not allow for any legitimate exception. They do not leave room, in any morally acceptable way, for the "creativity" of any contrary determination whatsoever. Once the moral species of an action prohibited by a universal rule is concretely recognized, the only morally good act is that of obeying the moral law and of refraining from the action which it forbids." (Veritatis Splendor, n. 67).

Intrinsically evil acts are always immoral, and are never justified by intention, or by circumstances, or by other knowingly chosen acts. A sexual act is any deliberate use of the genital sexual faculty.

Sexual acts are not exempt from the moral law. To be moral, each and every knowingly chosen sexual act must have three good fonts of morality. The intention must be good, the moral object must be good, and the good consequences must outweigh any bad consequences.

In order to have a good moral object, each and every sexual act must be marital and unitive and procreative. Each and every moral sexual act always has these three meanings: marital, unitive, and procreative. The deprivation of any one or more of these meanings from the moral object causes the sexual act to be intrinsically evil and always gravely immoral.

The natural sexual act is genital-to-genital intercourse between a man and a woman. This act is unitive and procreative. Natural sexual intercourse between a husband and wife is called natural marital relations. Only natural marital relations are martial and unitive and procreative.

The use of contraception deprives the act of natural intercourse of the procreative meaning, causing the sexual act to be non-procreative. The use of contraception is intrinsically evil and always gravely immoral because it deprives sexual relations of its procreative meaning, which is required by God for sexual acts to be moral. Therefore, natural marital relations must always be open to life (not contracepted).

A non-marital sexual act is any type of sexual act outside of marriage. Acts of adultery, pre-marital sex, and masturbation are non-marital. All non-marital sexual acts are intrinsically evil and always gravely immoral because these acts lack the marital meaning, which is required by God for sexual acts to be moral.

An unnatural sexual act is any type of sexual act that is not unitive and procreative. Examples of unnatural sexual acts include oral sexual acts, anal sexual acts, and manipulative sexual acts (i.e. masturbation of self or of another). All unnatural sexual acts are intrinsically evil and always gravely immoral because these acts lack the unitive and procreative meanings, which are required by God for sexual acts to be moral. These acts are not procreative because they are not the type of act that is inherently directed at procreation. These acts are not truly unitive, even if there is a certain mere physical union of body parts, because this is not the type of sexual union intended by God for human persons. Unnatural sexual acts are not justified by being done within marriage because the moral law requires each and every sexual act to be not only marital, but also unitive and procreative.

To be moral, each and every sexual act must be marital and unitive and procreative. All non-marital sexual acts, all non-unitive sexual acts, and all non-procreative sexual acts are intrinsically evil and always gravely immoral. All such acts have an evil moral object, and so they are not justified by intention, or by circumstances, or by other acts. Contraception deprives the sexual act of its procreative meaning, thereby causing the contracepted sexual act to be intrinsically evil and always gravely immoral. When a man and woman choose to deprive the sexual act of its procreative meaning, they are choosing to reject one of the inherent meanings of sexuality in the plan of God for human nature. This rejection is gravely immoral because "the moral order of sexuality involves such high values of human life that every direct violation of this order is objectively serious." (Sacred Congregation for the Doctrine of the Faith, Persona Humana, X.)

Pope Paul VI: "The Church, nevertheless, in urging men to the observance of the precepts of the natural law, which it interprets by its constant doctrine, teaches that each and every marital act must of necessity retain its intrinsic relationship to the procreation of human life." (Humanae Vitae, n. 11-12.)

The moral object of an act of natural marital relations is threefold: marital, unitive, and procreative. The deprivation of any one or more of these three meanings from a sexual act causes the moral object to be evil, and the act to be intrinsically evil and always gravely immoral. Premarital sex is intrinsically evil because it is non-marital. Unnatural sexual acts are intrinsically evil because they are non-unitive and non-procreative. Contracepted acts of natural intercourse are intrinsically evil because they are non-procreative.

Pope Pius XI: "But no reason, however grave, may be put forward by which anything intrinsically against nature may become conformable to nature and morally good. Since, therefore, the conjugal act is destined primarily by nature for the begetting of children, those who, in exercising it, deliberately frustrate its natural power and purpose, sin against nature and commit a deed which is shameful and intrinsically vicious." (Casti Connubii, n. 54.)

Contraception is intrinsically against the design for human nature chosen by God, whereby sexual relations is ordered toward procreation, through the union of man and woman in marriage. The deliberate deprivation of the procreative meaning from sexual relations is contrary to natural law, contrary to the definitive teaching of the Magisterium, and intrinsically immoral. (The phrase 'intrinsically vicious' is a translation of the Latin text 'intrinsece inhonestum,' which is perhaps better translated as 'intrinsically immoral'.)

Pope John Paul II: "When couples, by means of recourse to contraception, separate these two meanings that God the Creator has inscribed in the being of man and woman and in the dynamism of their sexual communion, they act as arbiters of the divine plan and they manipulate and degrade human sexuality -- and with it themselves and their married partner -- by altering its value of total self-giving. Thus the innate language that expresses the total reciprocal self-giving of husband and wife is overlaid, through contraception, by an objectively contradictory language, namely, that of not giving oneself totally to the other. This leads not only to a positive refusal to be open to life but also to a falsification of the inner truth of conjugal love, which is called upon to give itself in personal totality." (Familiaris Consortio, n. 32.)

The two meanings inscribed by God in the being of man and woman, for use only within marriage, are the unitive and procreative meanings. In order to be moral, each and every marital sexual act must be unitive and procreative. The use of contraception separates the unitive and procreative meanings, depriving the sexual act of a good intended by God in His divine plan. The use of contraception closes the sexual act to life, and is therefore immoral.

Pope John Paul II, writing about Pope Paul VI's encyclical Humanae Vitae: "And he concluded by re-emphasizing that there must be excluded as intrinsically immoral 'every action which, either in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible.' “(Familiaris Consortio, n. 32, quoting Humanae Vitae, n. 14.)

All contraceptive acts deliberately render the use of the sexual faculty non-procreative. Contraceptive acts done "in anticipation of the conjugal act" would include taking a contraceptive pill or applying a contraceptive barrier before sexual relations. Contraceptive acts done in the "accomplishment" of the sexual act would include the withdrawal method of contraception, and any inherently non-procreative sexual act (unnatural sexual acts). Contraceptive acts done "in the development of its [the conjugal act's] natural consequences" would include methods that interfere with conception after intercourse, such as spermicides and pills that prevent ovulation.

Since the use of contraception is intrinsically evil, no intention and no circumstance can justify its use. Intrinsically evil are always immoral, even with good intentions, even in dire circumstances (Veritatis Splendor, n. 81). The use of contraception, even by married persons, is always gravely immoral.

Pope Paul VI: "From this it follows that they are not free to act as they choose in the service of transmitting life, as if it were wholly up to them to decide what is the right course to follow. On the contrary, they are bound to ensure that what they do corresponds to the will of God the Creator. The very nature of marriage and its use makes His will clear, while the constant teaching of the Church spells it out." (Humanae Vitae, n. 10.)

The only moral sexual act is natural marital relations open to life. The use of contraception is an objective mortal sin because it closes the sexual act to life. The use of contraception with full knowledge that the act is gravely immoral, and with full deliberation, is an actual mortal sin. The use of contraception deprives the sexual act of the procreative meaning and is therefore intrinsically evil and always gravely immoral. When an act is intrinsically evil, neither a good intention, nor dire circumstances, can cause the act to become moral.

Catechism of the Catholic Church: "Legitimate intentions on the part of the spouses do not justify recourse to morally unacceptable means (for example, direct sterilization or contraception)." (CCC, n. 2399)

The end does not justify the means. And so the intended end of preventing disease transmission does not justify the use of an intrinsically evil means, contraception. Acts which are not intrinsically evil may be moral, depending on intention and circumstances. But acts which are intrinsically evil are always immoral, regardless of intention and circumstances.

Pope John Paul II: "But the negative moral precepts, those prohibiting certain concrete actions or kinds of behavior as intrinsically evil, do not allow for any legitimate exception. They do not leave room, in any morally acceptable way, for the 'creativity' of any contrary determination whatsoever. Once the moral species of an action prohibited by a universal rule is concretely recognized, the only morally good act is that of obeying the moral law and of refraining from the action which it forbids." (Veritatis Splendor, n. 67.)

The moral species is the type of act in terms of morality; it is the essential nature of the act according to the eternal moral law of God. Contraception is always gravely illicit because it is a type of act that is inherently contrary to the law of God on human sexuality. The deprivation of the procreative meaning causes the moral object of contracepted sexual acts to be evil, and the act of using contraception to be, in and of itself, gravely illicit. Even good intentions and dire circumstances cannot cause an inherently illicit act to become moral or justifiable.

The Catechism of the Catholic Church: "It is therefore an error to judge the morality of human acts by considering only the intention that inspires them or the circumstances (environment, social pressure, duress or emergency, etc.) which supply their context. There are acts which, in and of themselves, independently of circumstances and intentions, are always gravely illicit by reason of their object; such as blasphemy and perjury, murder and adultery. One may not do evil so that good may result from it." (CCC, n. 1756.)

Pope John Paul II: "No circumstance, no purpose, no law whatsoever can ever make licit an act which is intrinsically illicit, since it is contrary to the Law of God which is written in every human heart, knowable by reason itself, and proclaimed by the Church." (Evangelium Vitae, n. 62.) A medical purpose (first font) cannot transform the moral object (second font) of the act of using contraception from an evil to good. The use of contraception deprives the sexual act of its procreative meaning, making the moral object and the intrinsic moral meaning of the act evil. When the act of using contraception is done for a good purpose (or intention), the moral object of the act remains evil. Intentions and circumstances cannot change the moral object. The use of contraception is intrinsically evil and always gravely immoral, even when used for a medical purpose, or in dire circumstances.

A good end does not justify an intrinsically evil means.

[Romans]
{3:8} And should we not do evil, so that good may result? For so we have been slandered, and so some have claimed we said; their condemnation is just.

Sacred Congregation for the Doctrine of the Faith: "It is never permitted to do something which is intrinsically illicit, not even in view of a good result: the end does not justify the means." (Dignitas Personae, n. 21.)

Pope John Paul II: "…the end never justifies the means." (Message for the Celebration of the World Day of Peace, 1 January 2004, n. 8.)

Pontifical Council for the Family: "…one cannot do evil for a good end. The end does not justify the means." (Cardinal Alfonso López Trujillo, Pontifical Council for the Family, 3. c.)

Catechism of the Catholic Church: “‘An evil action cannot be justified by reference to a good intention' (cf. St. Thomas Aquinas, Dec. praec. 6). The end does not justify the means." (CCC, n. 1759; inner quote from St. Thomas Aquinas, On the Ten Commandments.)

The use of an intrinsically evil means is never justified by a good purpose (i.e. a good intention). The use of contraception is intrinsically evil because it deprives the sexual act of its procreative meaning. A medical purpose (the intended end) can never justify the use of an intrinsically evil means to achieve that end.

Both Pope Paul VI and Pope John Paul II taught that the use of contraception is intrinsically evil and always gravely immoral, "whether as an end or as a means." (Familiaris Consortio, n. 32, quoting Humanae Vitae, n. 14.)

A contraceptive act is an end when the purpose (the intention or intended end) is to prevent conception. But even if the contraceptive is used as a means to another end, such as when chemical contraceptives are used to treat a medical problem, or when a barrier method is used to prevent disease transmission, the use of contraception remains intrinsically evil. The intended end is in the first font; the effects (consequences) are in the third font. But the act of using contraception remains inherently directed at an evil moral object: the deprivation of the procreative meaning from a sexual act. A good intended end and good anticipated consequences cannot change the inherent moral meaning of the act itself. Contraceptive acts are inherently immoral.